1. Field of Invention
The present invention relates to the field of marine seismic data acquisition systems and methods of using same. More specifically, the invention relates to marine seismic data acquisition systems comprising a grid of seismic instruments bound by a controllable boundary, and methods for acquiring surveys using same.
2. Related Art
The performance of a marine seismic acquisition survey typically involves one or more vessels towing at least one seismic streamer through a body of water believed or known to overlie one or more hydrocarbon-bearing formations. WesternGeco currently conducts high-resolution Q-Marine™ surveys, in some instances covering many square kilometers. A survey vessel known as a Q-Technology™ vessel may conduct seismic surveys towing multiple, 1000-12000-meter cables with a separation of 25-200 meters, using the WesternGeco proprietary calibrated Q-Marine™ source. “Q” is the WesternGeco proprietary suite of advanced seismic technologies for enhanced reservoir location, description, and management. For additional information on Q-Marine™, a fully calibrated, point-receiver marine seismic acquisition and processing system, as well as Q-Land™ and Q-Seabed™, see http://www.westerngeco.com/q-technology. The seismic vessel and streamers progress forward at about 5 knots and the system is able to cover large areas of open ocean relatively efficiently. Thus, the traditional towed streamer seismic acquisition system is well-suited to explore the geological structures of previously unexplored or unexploited areas.
With the advent of 4-D marine seismic acquisition the need for improved seismic acquisition systems and techniques has been identified. 4-D seismic acquisition has time as the fourth dimension. This means that repeated seismic acquisition is made to determine how the reservoir characteristics evolve with time, mainly during production from the reservoirs and during water and gas injections, etc. 4-D seismic acquisition puts stronger requirements on resolution and re-position accuracy, leading to new possibilities in operational aspects. In contrast to 2-D and 3-D surveying of large areas of ocean, 4-D seismic acquisition is performed at already identified oil and gas fields, which mean that the areas to be covered are small compared to the areas covered when one is searching for new oil and gas fields. The areas are often crowded with obstructions like platforms and vessels. Thus, there is a need in the art for an entirely new way of positioning streamers and acoustic sources in order to efficiently cover small areas with very high positioning accuracy. Most streamer systems today are only controlled at one end by the towing vessel. The rest of the system is left to feather with the current. WesternGeco's Q-streamer™ uses small fins, so-called Q-fins™, to laterally control the streamer, but still only 3 degrees of steered feather is achieved. In addition, positioning streamer systems in areas obstructed by offshore installations may be difficult and risky. Thus, even though streamer systems cover large areas and are towed relatively fast (4-5 knots), the time lost due to feather mis-match, turning onto next line, and interference with obstructions is so large that other more controllable systems may be economically feasible.
A good 4-D acquisition system should be able, in a cost effective manner, to record weaker 4-D signals than is possible with current technology. To do so it will have to be able to filter out the noise that may be of the same order as the signal itself. If this is possible this will lead to a demand for more frequent 4-D surveys and lead to a more continuous and accurate field management. To record weak signals a minimum of resolution is required and this puts restrictions on the maximum sensor spacing. On the other hand it is not necessarily the resolution of the 4-D signal that governs the required sensor spacing. Often the noise is larger than the 4-D signal and hence the resolution of the noise is governing. The main noise factors in 4-D marine seismic acquisition comprise mis-position errors, diffracted multiples, and flow noise and mechanical noise resulting from the fluid flow. The first two noise factors are artificial signals that come from failure to repeat sensor position accurately enough for repeating the signal from the geological structure and the diffracted multiples respectively. The noise becomes apparent once making difference plots between two surveys taken at different times. The third noise type is a type of noise that must be filtered out for every data gather. Mis-position noise has received considerable attention from seismic providers. 10 meter re-position accuracy is a number often given by the geophysicist as a requirement. If the lateral sensor spacing is of the order of the required re-position accuracy, then steering is not an issue except for the purpose of maintaining good coverage. In this high density system there will always be a sensor within the re-position acceptance band. However, with today's streamer technology 37.5 m seems to be a lower limit to how close together one can tow streamers without introducing excessive operational risk. This implies that streamer steering for the purpose of re-positioning the sensors is the only feasible way using today's technology. However, as the streamers are affected by currents, it turns out to be a game of managing current, a game one cannot win efficiently. Diffracted multiples are scattered noise associated with the reflected seismic signals. This effect has received little attention mainly because it puts even stricter requirements on the resolution and re-positioning. A re-position accuracy of about 5 meters is typically demanded. Multiples also refract in all directions which means that a uniform sensor grid may be required. Lastly, flow noise is one of the dominant noise factors. The sensor spacing required depends on the resolution of the noise structure one is looking at.
The offshore seismic business has traditionally been extremely cyclic with very good profits during upturns and huge losses during downturns. In an environment like this the winner is the one that is able to ramp up quickly when prices improve and ramp down quickly when prices drop, and in addition is able to minimize capital expenditures for non-productive equipment during downturns. A seismic vessel, with its maritime crew, is expensive to run, and until now each seismic vessel needed to be designed for the purpose of handling seismic equipment. Therefore the cost associated with the vessel must be included even during downturns. For future systems one should consider this aspect and consider designing the handling equipment in such a way that it can be temporarily placed onboard readily available multi-purpose vessels like supply- and stand by vessels. As opposed to previous days of exploration that took place far away from any infrastructure, 4-D surveys take place around known oil and gas fields. In these areas there already exists infrastructure like supply- and stand-by vessels. Many of these vessels are equipped with huge diesel-electric systems mainly for the purpose of being able to supply enough power during emergency tow situations, anchor handling, and the like. These vessels often also have huge deck area that can be used for storage of containerized handling equipment and containerized instrument rooms.
To achieve high density surveys in regions having a combination of imaging and logistical challenges, a high trace density and closely spaced streamers may be used, however, this presents the potential of entangling and damaging streamer cables and associated equipment, unless streamer steering devices are closely monitored and controlled. While the Q™ suite of advanced technologies for marine seismic data acquisition and processing may provide detailed images desired for many reservoir management decisions, including the ability to acquire wide- and/or full azimuth data, the capital expense may be higher than desired in certain situations. The ability to acquire marine seismic data using less capital intensive projects, while increasing the diversity of azimuth and offset, are constant goals of the marine seismic industry and would be viewed as advances in the art. U.S. Pat. No. 6,009,042 discloses towed grids having uniformly spaced hydrophones in a “fish net” or baked into a fabric, however, their motivation was different from the present invention and as such the reference does not disclose or suggest methods and systems of the present invention.